Nitride based semiconductor optical device, epitaxial wafer for nitride based semiconductor optical device, and method of fabricating semiconductor light-emitting device

ABSTRACT

In the nitride based semiconductor optical device LE 1,  the strained well layers  21  extend along a reference plane SR 1  tilting at a tilt angle α from the plane that is orthogonal to a reference axis extending in the direction of the c-axis. The tilt angle α is in the range of greater than 59 degrees to less than 80 degrees or greater than 150 degrees to less than 180 degrees. A gallium nitride based semiconductor layer P is adjacent to a light-emitting layer SP− with a negative piezoelectric field and has a band gap larger than that of a barrier layer. The direction of the piezoelectric field in the well layer W 3  is directed in a direction from the n-type layer to the p-type layer, and the piezoelectric field in the gallium nitride based semiconductor layer P is directed in a direction from the p-type layer to the n-type layer. Consequently, the valence band, not the conduction band, has a dip at the interface between the light-emitting layer SP− and the gallium nitride based semiconductor layer P.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 12/752,647filed on Apr. 1, 2010, which is a continuation of PCT application No.PCT/JP2009/050992 filed on Jan. 22, 2009, claiming the benefit ofpriorities from Japanese Patent application No. 2008-233806 filed onSep. 11, 2008, and incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a nitride based semiconductor opticaldevice, an epitaxial wafer for a nitride based semiconductor opticaldevice, and the method of fabricating a semiconductor light-emittingdevice.

BACKGROUND ART

Patent Document 1 discloses a semiconductor optical device. In thesemiconductor optical device, a piezoelectric field in a strained layerdoes not occur at all in a direction tilted at an angle of about 40degrees, 90 degrees, and 140 degrees defined with reference to the[0001] direction. Accordingly, a plane orientation is selected withinthe angle range of 30 to 50 degrees, 80 to 100 degrees, or 130 to 150degrees. Epitaxial growth occurs onto the surface of a substrate onwhich little or no piezoelectric field is generated in a strainedquantum-well structure.

Patent Document 2 discloses a semiconductor light-emitting device. Thissemiconductor light-emitting device is formed on a nonpolar plane. Thenonpolar plane encompasses a {11-20} plane, a plane tilted from the{11-20} plane at an angle in the range of −5 to +5 degrees, a {1-100}plane, or a plane tilted from the {1-100} plane at an angle in the rangeof −5 to +5 degrees.

Non-Patent Document 1 discloses a theoretical study of dependence of thepiezoelectric effect on crystal orientation in an InGaN/GaN heterostructure of a wurtzite structure. A strained layer grown in a crystalorientation at an off-angle of 39 degrees or 90 degrees with referenceto (0001) does not induce the longitudinal component of thepiezoelectric field. Furthermore, Non-Patent Document 2 discloses theeffect of crystal orientation in relation to electrical characteristicsof an InGaN/GaN quantum-well of a wurtzite structure. The internalelectric field of the InGaN/GaN quantum-well structure changes its signaround at an off-angle of about 55 degrees.

[Patent Document 1] Japanese Unexamined Patent Application PublicationNo. 11-112029 (Japanese Patent Application No. 09-263511)

[Patent Document 2] Japanese Unexamined Patent Application PublicationNo. 10-135576

[Non-Patent Document 1] Jpn. J. Appl. Phys., vol. 39 (2000), pp.413-416. Part 1. No. 2A, February

[Non-Patent Document 2] J. Appl. Phys., Vol. 91, No. 12, 15 June 2002,pp. 9904-9908.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

A significantly large piezoelectric field is generated in an InGaN welllayer grown on a (0001) plane of a gallium nitride based semiconductor.The piezoelectric field spatially separates the wave function ofelectrons from that of holes in an active layer, thereby loweringlight-emitting efficiency of a light-emitting device. Furthermore, inthe light-emitting device, carriers injected into the active layerscreen the piezoelectric field therein as the applied current isincreased. Such screening causes a blue shift of the emission wavelengthwith the increase in the applied current.

In Patent Document 2, in order to avoid a large blue shift, the activelayer is formed on a {11-20} plane or a {1-100} plane, which defines anangle of 90 degrees with the (0001) plane.

In Patent Document 1, in order to avoid a large blue shift, an off-angleof 40 degrees or 140 degrees, at which the internal electrical field ofthe active layer is zero, is used. In Non-Patent Document 1, theoff-angle at which the internal electrical field is zero is estimatedthrough theoretical calculation.

However, in order to fabricate a wafer having a primary surface of a{11-20} plane or a {10-10} plane, i.e., a nonpolar primary surface, aingot thick grown in the (0001) plane direction is cut out to form acrystal piece such that the primary surface has the plane orientationmentioned above. Since the ingot is cut out in its longitudinaldirection, the width of the cut-out crystal piece is about 10millimeters at most.

In Patent Documents 1 and 2, plane orientation that makes thepiezoelectric field zero or near zero is utilized. Inventors'investigations have revealed that, unlike the inventions of PatentDocuments 1 and 2, utilizing a piezoelectric field with a non-zeromagnitude can enhance performances of semiconductor light-emittingdevices.

It is an object of the present invention to provide a nitride basedsemiconductor optical device that includes a light-emitting layer of astrained hexagonal group III nitride and can suppress overflow ofelectrons from the light-emitting layer. It is another object of thepresent invention to provide an epitaxial wafer for the nitride basedsemiconductor optical device. Furthermore, it is yet another object ofthe present invention to provide a method of fabricating a semiconductorlight-emitting device that includes a light-emitting layer of a strainedhexagonal group III nitride.

Means for Solving the Problems

A nitride based semiconductor optical device according to one aspect ofthe present invention includes (a) a first gallium nitride basedsemiconductor region; (b) a light-emitting layer including a well layerand a barrier layer, the well layer being composed of a strainedhexagonal gallium nitride based semiconductor, and the barrier layerbeing composed of gallium nitride based semiconductor; and (c) a secondgallium nitride based semiconductor region. The light-emitting layer isprovided between the first gallium nitride based semiconductor regionand the second gallium nitride based semiconductor region. The firstgallium nitride based semiconductor region includes one or more n-typegallium nitride semiconductor layers. The second gallium nitride basedsemiconductor region includes a gallium nitride based semiconductorlayer and one or more p-type gallium nitride semiconductor layers, andthe gallium nitride based semiconductor layer has a band gap larger thanthat of the barrier layer. Each of the well layer and the barrier layerextends along a reference plane, and the reference plane tilts at a tiltangle in the range of 59 degrees to less than 80 degrees and greaterthan 150 degrees to less than 180 degrees from a plane orthogonal to areference axis extending in the direction of a c-axis. A piezoelectricfield in the light-emitting layer includes a component of a directionopposite to a direction from the second gallium nitride basedsemiconductor region toward the first gallium nitride basedsemiconductor region. The gallium nitride based semiconductor layer ofthe second gallium nitride based semiconductor region is adjacent to thelight-emitting layer; and the gallium nitride based semiconductor layerof the second gallium nitride based semiconductor region includes one ofan electron blocking layer and a cladding layer.

In the nitride based semiconductor optical device, since each of thewell layer and the barrier layer extends along a reference plane tiltingat a tilt angle within the above-mentioned range, the piezoelectricfield in the light-emitting layer includes a component of the directionthat is opposite to the direction from the second gallium nitride basedsemiconductor region toward the first gallium nitride basedsemiconductor region, whereas the piezoelectric field in the galliumnitride semiconductor layer includes a component of the direction thatis the same as the direction from the second gallium nitride basedsemiconductor region toward the first gallium nitride basedsemiconductor region. Since the light-emitting layer and the galliumnitride semiconductor layer of the second gallium nitride basedsemiconductor region are adjacent to each other, the valence band, notthe conduction band, has a dip at the interface between the galliumnitride semiconductor layer and the light-emitting layer. Accordingly,the conduction band has no dip thereat, so that overflow of electronscan be reduced.

In the nitride based semiconductor optical device according to thepresent invention, the well layer may be composed of InGaN, and thebarrier layer may be composed of GaN or InGaN. According to this nitridebased semiconductor optical device, the lattice constants in thedirections of the a-axis and the c-axis in InN are larger than those inthe directions of the a-axis and the c-axis in GaN, respectively.Consequently, the InGaN well layer is strained due to stress from thebarrier layer.

In the nitride based semiconductor optical device according to thepresent invention, the tilt angle may be in the range of 62 degrees toless than 80 degrees. This nitride based semiconductor optical deviceexhibits reduced blue shift. Alternatively, in the nitride basedsemiconductor optical device according to the present invention, thetilt angle may be in the range of greater than 150 degrees to 170degrees. This nitride based semiconductor optical device also exhibitsreduced blue shift.

The nitride based semiconductor optical device according to the presentinvention may include a substrate composed of a hexagonal semiconductorIn_(S)Al_(T)Ga_(1-S-T)N (0≦S≦1, 0≦T≦1, 0≦S+T≦1). The primary surface ofthe substrate extends along a plane tilting at a tilt angle in the rangeof greater than 59 degrees to less than 80 degrees or greater than 150degrees to less than 180 degrees with reference to a plane orthogonal tothe c-axis of the hexagonal semiconductor. The first gallium nitridebased semiconductor region, the light-emitting layer, and the secondgallium nitride based semiconductor region are arranged on the primarysurface of the substrate in the direction of a predetermined axis.

In the nitride based semiconductor optical device, by use of the abovesubstrate, the piezoelectric field in the light-emitting layer includesa component of the direction that is opposite to the direction from thesecond gallium nitride based semiconductor region toward the firstgallium nitride based semiconductor region.

In the nitride based semiconductor optical device according to thepresent invention, the substrate includes a plurality of first regionsand a plurality of second regions. The first regions has the density ofthreading dislocations, which is larger than a first threadingdislocation density, extending in the c-axis direction, and the secondregions has the density of threading dislocations, which is smaller thanthe first threading dislocation density, extending in the c-axisdirection. The first and second regions are alternately arranged and areexposed at the primary surface of the substrate.

In this nitride based semiconductor optical device, since thelight-emitting layer is grown on the second regions, the light emissioncharacteristics are less affected by the threading dislocation densitythereof.

In the nitride based semiconductor optical device according to thepresent invention, the second regions can have a threading dislocationdensity of less than 1×10⁷ cm⁻². This nitride based semiconductoroptical device can provide a light-emitting layer having excellent lightemission characteristics.

In the nitride based semiconductor optical device according to thepresent invention, the first gallium nitride based semiconductor region,the light-emitting layer, and the second gallium nitride basedsemiconductor region constitute a semiconductor laminate, which isprovided on the primary surface of the substrate, and the substrate haselectrical conductivity. The hexagonal nitride based semiconductoroptical device may include a first electrode provided on thesemiconductor laminate and a second electrode provided on the back sideof the substrate. According to the nitride based semiconductor opticaldevice, both of the anode and the cathode not are provided on the sameupper face of the epitaxial laminate.

In the nitride based semiconductor optical device according to thepresent invention, the light-emitting layer may include first and secondoptical guide layers and an active layer having a quantum-wellstructure, and the quantum-well structure may include the well layer andthe barrier layer. The active layer may be provided between the firstoptical guide layer and the second optical guide layer. This nitridebased semiconductor optical device can provide a semiconductor laser.

In the nitride based semiconductor optical device according to thepresent invention, the reference plane may tilt toward the direction ofthe a-axis. The tilt toward the direction of the a-axis allows m-planecleavage in the nitride based semiconductor optical device.

In the nitride based semiconductor optical device according to thepresent invention, the reference plane may tilt toward the direction ofthe m-axis. The tilt toward the direction of the m-axis allows m-planecleavage in the nitride based semiconductor optical device.

In the nitride based semiconductor optical device according to thepresent invention, the gallium nitride based semiconductor layer of thesecond gallium nitride based semiconductor region may be composed ofp-type Al_(X)Ga_(Y)In_(1-X-Y)N (0<X≦1, 0≦Y≦1, 0<X+Y≦1) that contains atleast aluminum. This nitride based semiconductor optical device canefficiently confine carriers into the light-emitting layer.

Another aspect of the present invention relates to an epitaxial waferfor a nitride based semiconductor optical device. The epitaxial wafercomprises: (a) a first gallium nitride based semiconductor region; (b) alight-emitting layer including a strained well layer and a barrierlayer, the well layer being composed of a hexagonal gallium nitridebased semiconductor, and the barrier layer being composed of a galliumnitride based semiconductor; (c) a second gallium nitride basedsemiconductor region; and (d) a wafer of a hexagonal semiconductor ofIn_(S)Al_(T)Ga_(1-S-T)N (0≦S≦1, 0≦T≦1, 0≦S+T≦1). The light-emittinglayer is provided between the first gallium nitride based semiconductorregion and the second gallium nitride based semiconductor region on thewafer. The first gallium nitride based semiconductor region includes oneor more n-type gallium nitride semiconductor layers. The second galliumnitride based semiconductor region includes a gallium nitride basedsemiconductor layer and one or more p-type gallium nitride basedsemiconductor layers, and the gallium nitride based semiconductor layerhas a band gap larger than that of the barrier layer. Each of the welllayer and the barrier layer extends along a reference plane, and thereference plane tilts at a tilt angle in a range of one of 59 degrees toless than 80 degrees and greater than 150 degrees to less than 180degrees from a plane orthogonal to a reference axis extending in adirection of a c-axis. A piezoelectric field in the light-emitting layerincludes a component of a direction opposite to a direction from thesecond gallium nitride based semiconductor region toward the firstgallium nitride based semiconductor region. The gallium nitride basedsemiconductor layer of the second gallium nitride based semiconductorregion is adjacent to the light-emitting layer, and the gallium nitridebased semiconductor layer of the second gallium nitride basedsemiconductor region includes one of an electron blocking layer and acladding layer. The barrier layer comprises one of GaN and InGaN.

In this epitaxial wafer, since each of the well layer and the barrierlayer extend along the reference plane that tilts at a tilt angle in theabove-mentioned range, the piezoelectric field in the light-emittinglayer includes a component in the direction that is opposite to thedirection from the second gallium nitride based semiconductor regiontoward the first gallium nitride based semiconductor region. On theother hand, the piezoelectric field in the gallium nitride basedsemiconductor layer includes a component in the direction that is thesame as the direction from the second gallium nitride basedsemiconductor region toward the first gallium nitride basedsemiconductor region. Since the gallium nitride based semiconductorlayer of the second gallium nitride based semiconductor region is incontact with the light-emitting layer, a dip is formed in the valenceband, not the conduction band, at the interface between the galliumnitride semiconductor layer and the light-emitting layer. The dip in thevalence band, not the conductive band, can reduce overflow of electrons.

In the epitaxial wafer according to the present invention, the primarysurface of the wafer may extend along a plane tilting at a tilt angle inthe range of 59 degrees to less than 80 degrees or greater than 150degrees to 170 degrees with reference to a plane orthogonal to thec-axis of the hexagonal semiconductor. According to the epitaxial wafer,the well layer and the barrier layer are provided so as to extend alongrespective reference planes each tilting at a tilt angle in theabove-mentioned range by adjusting the tilt angle of the primary surfaceof the wafer within the above-mentioned range.

In the epitaxial wafer according to the present invention, the maximumdistance between two points on the edge of the wafer may be 45millimeters or more. According to the epitaxial wafer, a wafer with alarge diameter can be provided, unlike those of a primary surface of thea-plane or the m-plane. In the epitaxial wafer according to the presentinvention, the wafer may be composed of electrically conductive GaN.

In the method according to the present invention, the first galliumnitride based semiconductor region, the light-emitting layer, and thesecond gallium nitride based semiconductor region are arranged on theprimary surface of the wafer in the direction of a predetermined axis.The direction of the reference axis is different from the direction ofthe predetermined axis. In this method, the direction of the abovearrangement is the same as the direction of a predetermined axis, andthe epitaxial growth occurs in the direction of the reference axis.

In the epitaxial wafer according to the present invention, the tiltangle may be in the range of 62 degrees to less than 80 degrees. Thisepitaxial wafer can procide a nitride based semiconductor optical devicewith a small blue shift. Alternatively, in the epitaxial wafer accordingto the present invention, the tilt angle may be in the range of greaterthan 150 degrees to less than 170 degrees. This epitaxial wafer can alsoprovide a nitride based semiconductor optical device with a small blueshift.

Yet another aspect of the present invention relates to a method offabricating a semiconductor light-emitting device, and the semiconductorlight-emitting device includes a light-emitting layer of a strainedhexagonal group III nitride. The method comprises the steps of: (a)choosing a plane orientation for the light-emitting layer for estimatinga direction of a piezoelectric field in the light-emitting layer; (b)forming a quantum well structure for estimating the direction of thepiezoelectric field in the light-emitting layer, and p-type and n-typegallium nitride semiconductors to prepare a substrate product, thequantum well structure being formed in the chosen plane orientation; (c)measuring photoluminescence of the substrate product while applyingvoltage across the substrate product, to obtain a voltage dependency ofphotoluminescence; (d) estimating the direction of the piezoelectricfield in the light-emitting layer based on the measured voltagedependency; (e) preparing a wafer having a primary surface for formingthe light-emitting layer with the chosen plane orientation; and (f)forming a semiconductor laminate for the semiconductor light-emittingdevice on the primary surface of the wafer. The semiconductor laminateincludes a first gallium nitride based semiconductor region, thelight-emitting layer, and a second gallium nitride based semiconductorregion. The light-emitting layer includes a well layer of galliumnitride based semiconductor and a barrier layer of gallium nitride basedsemiconductor, and the light-emitting layer is provided between thefirst gallium nitride based semiconductor region and the second galliumnitride based semiconductor region. The first gallium nitride basedsemiconductor region includes one or more n-type gallium nitride basedsemiconductor layers. The second gallium nitride based semiconductorregion includes a gallium nitride based semiconductor layer and one ormore p-type gallium nitride based semiconductor layers, and the galliumnitride based semiconductor layer has a band gap larger than that of thebarrier layer. The gallium nitride based semiconductor layer in thesecond gallium nitride based semiconductor region is adjacent to thelight-emitting layer. Each of the well layer and the barrier layerextends along a reference plane, and the reference plane tilts from aplane orthogonal to a reference axis extending in a direction of each ofa c-axis, the a-axis and the m-axis. A direction of the piezoelectricfield is defined with reference to a direction from the second galliumnitride based semiconductor region toward the first gallium nitridebased semiconductor region.

In this method, in order to estimate the dependency of PL on biasvoltage, photoluminescence (PL) spectra of the substrate product aremeasured while voltage is applied to the substrate product. These PLspectra can be measured in the range of positive and negative voltagesthat are smaller than the applied voltage at which emission of light iscaused by electroluminescence (EL). The bias dependency of PL spectraprovides estimation of the magnitude and direction of the internalelectrical field in the light-emitting layer. This estimation can beutilized to form a light-emitting device with a piezoelectric field in adesired direction.

Advantages

As described above, one aspect of the present invention provides anitride based semiconductor optical device that includes alight-emitting layer of a strained hexagonal group III nitride and cansuppress overflow of electrons from the light-emitting layer. Inaddition, another aspect of the present invention provides an epitaxialwafer for this nitride based semiconductor optical device. A furtheraspect of the present invention provides a method of fabricating asemiconductor light-emitting device including a light-emitting layer ofa strained hexagonal group III nitride.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other objects, features, and advantages of the presentinvention will become more readily apparent from the following detaileddescription of the preferred embodiments of the present invention withreference to the accompanying drawings.

FIG. 1 is a schematic diagram showing the structure of a nitride basedsemiconductor optical device according to an embodiment of the presentinvention.

FIG. 2 is a view illustrating the directions of piezoelectric fields instrained light-emitting layers.

FIG. 3 is a view illustrating the directions of piezoelectric fields instrained light-emitting layers.

FIG. 4 is a schematic diagram showing the structure of a nitride basedsemiconductor optical device according to an embodiment of the presentinvention.

FIG. 5 is a view showing primary steps in the methods of fabricating anitride based semiconductor optical device according to an embodimentand an epitaxial wafer.

FIG. 6 is a view showing primary steps in the methods of fabricating anitride based semiconductor optical device according to an embodimentand an epitaxial wafer.

FIG. 7 is a view showing primary steps in the methods of fabricating anitride based semiconductor optical device according to an embodimentand an epitaxial wafer.

FIG. 8 is a diagram showing a structure of a GaN substrate that can beused in the embodiments according to the present invention.

FIG. 9 is a flowchart showing the method of estimating the direction andmagnitude of a piezoelectric field in a well layer.

FIG. 10 is a view illustrating the measurement of bias-dependence of PL.

FIG. 11 is a view showing observed EL spectra of a semiconductorlight-emitting device fabricated in an example.

FIG. 12 is a graph showing temperature dependency of integratedintensity.

FIG. 13 is a graph showing electroluminescence (EL) spectra measured atan absolute temperature of 300 K.

FIG. 14 is a graph showing EL spectra measured at an absolutetemperature of 10 K.

FIG. 15 is a diagram showing the structure of a semiconductor laserfabricated in an example.

REFERENCE NUMERALS

-   LE1, LD1: nitride based semiconductor optical device,-   13: first gallium nitride based semiconductor region,-   15: light-emitting region,-   17: second gallium nitride based semiconductor region,-   19: active layer,-   21: well layer,-   23: barrier layer,-   25, 27, 29: gallium nitride based semiconductor layer,-   31: gallium nitride based semiconductor layer,-   33, 35: gallium nitride based semiconductor layer,-   α, β: tilt angle,-   SR1: reference plane,-   B1, B2, B3, B4, B5, B6: barrier layer,-   W1, W2, W3: well layer,-   P, NP, SP+, SP−: light-emitting layer,-   37: undoped GaN layer (N₂—GaN layer),-   39: undoped GaN layer,-   41 a, 41 b: electrode,-   VC1, VC2: vector that indicates the c-axis direction,-   51, 53: gallium nitride based semiconductor layer,-   55, 57: gallium nitride based semiconductor layer,-   59 a, 59 b: optical guide layer,-   61: undoped GaN layer,-   63: insulating film,-   65, 67: electrode.

BEST MODES FOR CARRYING OUT THE INVENTION

The teachings of the present invention can be readily understood inconsideration of the following detailed description with reference toaccompanying drawings shown as examples. Subsequently, with reference tothe accompanying drawings, embodiments of the nitride basedsemiconductor optical device, the epitaxial wafer for the nitride basedsemiconductor optical device, and the method of fabricating thesemiconductor light-emitting device will be described. The same portionsare denoted by the same reference numerals, if possible.

FIG. 1 is a schematic diagram showing the structure of a nitride basedsemiconductor optical device according to an embodiment. The nitridebased semiconductor optical device encompasses, for example, asemiconductor laser and a light-emitting diode. In FIG. 1, a coordinatesystem S is shown. The primary surface 11 a of a substrate 11 isdirected to the Z-axis direction and extends in the X-axis and Y-axisdirections. The X-axis is directed to in the a-axis direction. In thefollowing description, for example, the crystal axis in opposite to the<0001> axis is represented by <000-1>.

A nitride based semiconductor optical device LE1 has a structuresuitable for a light-emitting diode. The nitride based semiconductoroptical device LE1 includes a first gallium nitride based semiconductorregion 13, a light-emitting region 15, and a second gallium nitridebased semiconductor region 17. The light-emitting layer 15 includes anactive layer 19, and the active layer 19 includes a well layer 21 and abarrier layer 23, which are alternately arranged. The light-emittinglayer 15 is provided between the first gallium nitride basedsemiconductor region 13 and the second gallium nitride basedsemiconductor region 17. The first gallium nitride based semiconductorregion 13 can include one or more n-type gallium nitride basedsemiconductor layers (in the present embodiment, gallium nitride basedsemiconductor layers 25, 27, and 29). The second gallium nitride basedsemiconductor region 17 includes a gallium nitride based semiconductorlayer 31 having a band gap larger than that of the barrier layer, andalso includes one or more p-type gallium nitride based semiconductorlayers (in the present embodiment, gallium nitride based semiconductorlayers 33 and 35).

In the nitride based semiconductor optical device LE1, the well layer 21extends along a reference plane SR1 tilting at a tilt angle α withrespect to the plane that is orthogonal to a reference axis (shown byvector VC1), which extends in the c-axis direction. The tilt angle α canbe in the range of 59 degrees to less than 80 degrees. Alternatively,the tilt angle α can be in the range of greater than 150 degrees to lessthan 180 degrees. The well layer 21 is strained, and the piezoelectricfield of the well layer 21 includes a component of the direction that isopposite to the direction from the second gallium nitride basedsemiconductor region 17 toward the first gallium nitride basedsemiconductor region 13. The gallium nitride based semiconductor layer31 of the second gallium nitride based semiconductor region 17 isadjacent to the light-emitting layer 15. The well layer 21 may becomposed of a hexagonal gallium nitride based semiconductor, forexample, a gallium nitride based semiconductor containing indium, suchas InGaN. The barrier layer 23 may be composed of a gallium nitridesemiconductor, for example, GaN, InGaN, AlGaN, or AlGaInN.

In this nitride based semiconductor optical device LE1, since each ofthe well layer 21 and the barrier layer 23 extends along the referenceplane SR1 tilting at a tilt angle α within the above-mentioned range,the piezoelectric field in the well layer 21 includes a component of thedirection (positive direction of the Z-axis) that is opposite to thedirection from the second gallium nitride based semiconductor region 17toward the first gallium nitride based semiconductor region 13, whereasthe piezoelectric field in the gallium nitride based semiconductor layer31 of the second gallium nitride based semiconductor region 17 includesa component of the direction (negative direction of the Z-axis) that isthe same as the direction from the second gallium nitride basedsemiconductor region 17 toward the first gallium nitride basedsemiconductor region 13. Since the gallium nitride based semiconductorlayer 31 is adjacent to the light-emitting layer 15, a dip is formed ina valence band, not in a conduction band, at the interface J1 betweenthe gallium nitride based semiconductor layer 31 and the light-emittinglayer 15. Such a dip not in the conduction band but in the valence bandcan reduce overflow of electrons.

The gallium nitride based semiconductor layer 31 of the second galliumnitride based semiconductor region 17 includes either an electronblocking layer or a cladding layer. The electron blocking layer blockselectrons from the active layer, and the cladding layer confines carriesand light. The gallium nitride based semiconductor layer 31 of thesecond gallium nitride based semiconductor region 17 can be composed of,for example, p-type AlGaN.

Since the lattice constants in the a-axis and c-axis directions of InNare larger than those in the a-axis and c-axis directions of GaN,respectively, the well layer 21 of, for example, InGaN incorporatesinternal strain due to stress (compressive stress) from the barrierlayer.

The tilt angle α can be in the range of 62 degrees to less than 80degrees. This nitride based semiconductor optical device can reduce theblue shift. Alternatively, the tilt angle α can be in the range ofgreater than 150 degrees to 170 degrees. This nitride basedsemiconductor optical device can reduce the blue shift.

FIG. 2 shows illustrations strained light-emitting layers and thedirections of piezoelectric fields therein. Parts (a) to (c) of FIG. 2are views illustrating piezoelectric fields in light-emitting layersformed on a polar plane (c-plane). Parts (d) and (e) of FIG. 2 are viewsillustrating piezoelectric fields in light-emitting layers formed on anonpolar plane (a-plane, m-plane). Parts (f) and (g) of FIG. 2 are viewsillustrating piezoelectric fields in light-emitting layers formed on asemipolar plane.

With respect to part (a) of FIG. 2, the light-emitting layer P includesbarrier layers B1 and B2 and a well layer W1 which are formed on thepolar plane (c-plane). The well layer W1 is provided between the barrierlayers B1 and B2. The piezoelectric field E_(PZ) in the well layer W1 isoriented in the direction from the p-type layer to the n-type layer. Inthe well layer, the band bottom of the conduction band and the bandbottom of the valence electrons descend in the direction from the n-typelayer toward the p-type layer. The reference symbol E_(C0) denotes anenergy difference between the band bottom of the conduction band and theband bottom of the valence band. With respect to part (b) of FIG. 2, alow forward-biased voltage is applied to the light-emitting layer P.This voltage application increases the tilts of the band bottom of theconduction band and the band bottom of the valence band in thislight-emitting layer P. The reference symbol E_(C1) denotes an energydifference between the band bottom of the conduction band and the bandbottom of the valence band. The energy difference E_(C0) is larger thanthe energy difference E_(C1). With respect to part (c) of FIG. 2, aforward-biased high voltage is applied to the light-emitting layer P. Inthis voltage application, the tilts of the band bottom of the conductionband and the band bottom of the valence band decrease due to screeningin this light-emitting layer P. The reference symbol E_(C2) denotes anenergy difference between the band bottom of the conduction band and theband bottom of the valence band. The energy difference E_(C2) is largerthan the energy difference E_(C0). Change in the energy differencecaused by the application of voltage leads to a blue shift.

As shown in part (d) of FIG. 2, the light-emitting layer NP includesbarrier layers B3 and B4 and a well layer W2, which are formed on thenonpolar plane (a-plane, m-plane). The well layer W2 is provided betweenthe barrier layers B3 and B4. Since the well layer W2 is grown on anonpolar plane, the piezoelectric field E_(PZ) is zero. In the welllayer W2, the band bottom of the conduction band and the band bottom ofthe valence band descend in the direction from the p-type layer towardthe n-type layer. The reference symbol E_(NP0) denotes an energydifference between the band bottom of the conduction band and the bandbottom of the valence band. With respect to part (e) of FIG. 2, aforward-biased voltage is applied to the light-emitting layer NP. Inthis light-emitting layer NP, the tilts of the band bottom of theconduction band and the band bottom of the valence band are almostcancelled by this voltage application. The reference symbol E_(NP1)denotes an energy difference between the band bottom of the conductionband and the band bottom of the valence electrons. The energy differenceE_(NP0) is smaller than the energy difference E_(NP1). Since thepiezoelectric field is zero in the light-emitting layer NP, screeningdoes not occur even if the amount of carriers in the well layerincreases. Accordingly, the application of voltage does not cause anychange in the energy difference, so that no blue shift is observed.

With respect to part (f) of FIG. 2, the light-emitting layer SP−includes barrier layers B5 and B6 and a well layer W3, which are formedon a semipolar plane tilting at a specific off-angle. The well layer W3is provided between the barrier layers B5 and B6. Since the well layerW3 is formed on a semipolar plane, the piezoelectric field E_(PZ) issmaller than that on a polar plane. In the well layer W3, the bandbottom of the conduction band and the band bottom of the valence banddescend in the direction from the p-type layer toward the n-type layer.The reference symbol E_(SP0) denotes an energy difference between theband bottom of the conduction band and the band bottom of the valenceband. With reference to part (g) of FIG. 2, a forward-biased voltage isapplied to the light-emitting layer SP−. In this light-emitting layerSP−, the application of the above voltage decreases the tilts of theband bottom of the conduction band and the band bottom of the valenceband. The reference symbol E_(NP1) denotes an energy difference betweenthe band bottom of the conduction band and the band bottom of thevalence band, and the energy difference E_(SP0) is larger than theenergy difference E_(SP1). Since the piezoelectric field in thelight-emitting layer SP− includes a component of the direction that isopposite to the direction from the p-type layer to the n-type layer, noscreening occurs. Therefore, the application of voltage causes a smallchange in the energy difference, so that the blue shift is very small.

The well layer (light-emitting layer SP−) of plane orientation of thetilt angle range in the present embodiment acts as shown in parts (f)and (g) of FIG. 2. But, the well layer (light-emitting layer SP+) on asemipolar plane which is different from the plane orientation in thetilt angle range according to the present embodiment acts as shown inparts (a) to (c) of FIG. 2.

Next, the light-emitting layer formed on a semipolar plane will befurther described below. FIG. 3 includes views illustrating strainedlight-emitting layers and the directions of piezoelectric fieldstherein. Parts (a) and (b) of FIG. 3 show light-emitting layers SP+incorporating positive piezoelectric fields. The light-emitting layerSP+ includes barrier layers B7 and B8 and a well layer W4. The welllayer W4 is provided between the barrier layers B7 and B8. A galliumnitride based semiconductor layer P, adjacent to the light-emittinglayer SP+, is shown which has a band gap larger than that of the barrierlayer. The gallium nitride based semiconductor layer P can be, forexample, a p-type electron blocking layer or a p-type cladding layer.The direction of the piezoelectric field in the well layer W4 is fromthe p-type layer toward the n-type layer, whereas the direction of thepiezoelectric field in the gallium nitride based semiconductor layer Pis from the n-type layer to the p-type layer. The conduction bandtherefore has a dip DIP1 at the interface between the light-emittinglayer SP+ and the gallium nitride based semiconductor layer P. This DIP1reduces the electronic barrier in the gallium nitride basedsemiconductor layer P. The height of dip DIP1 is, for example, about 0.2eV.

Parts (c) and (d) of FIG. 3 show a light-emitting layer SP− having anegative piezoelectric field, and in the figure, the gallium nitridebased semiconductor layer P having a band gap larger than those of thebarrier layers is adjacent to the light-emitting layer SP−. Thedirection of the piezoelectric field in the well layer W3 is from then-type layer toward the p-type layer, whereas the direction of thepiezoelectric field in the gallium nitride based semiconductor layer Pis from the p-type layer toward the n-type layer. Accordingly, a dip isformed in the valence band, not the conduction band at the interfacebetween the light-emitting layer SP− and the gallium nitride basedsemiconductor layer P. Consequently, the dip DIP2 of the conduction banddoes not lower the barrier to electrons flowing in the light-emittinglayer, and, therefore, the gallium nitride based semiconductor layer Pcan work as a sufficient blocker to the electrons in the light-emittinglayer. The height of the dip DIP2 is, for example, about 0.1 eV.

Referring to FIG. 1 again, the semiconductor light-emitting device LE1will be described below. The n-type gallium nitride based semiconductorlayer 25 in the first gallium nitride based semiconductor region 13 canbe a Si-doped n-type AlGaN buffer layer having a thickness of, forexample, 50 nanometers. The n-type gallium nitride based semiconductorlayer 27 can be a Si-doped n-type GaN layer having a thickness of, forexample, 2000 nanometers. The n-type gallium nitride based semiconductorlayer 29 can be a Si-doped n-type InGaN buffer layer, and the indiumfraction thereof is, for example, 0.02. The thickness of the n-typegallium nitride based semiconductor layer 29 can be, for example, 100nanometers.

The p-type gallium nitride based semiconductor layer 31 of the secondgallium nitride based semiconductor region 17 is, for example, aMg-doped p-type AlGaN layer, and its fraction of aluminum is, forexample, 0.07. The thickness of the p-type gallium nitride basedsemiconductor layer 31 is, for example, 20 nanometers. The p-typegallium nitride based semiconductor layer 33 can be a Mg-doped p-typeGaN layer, and its thickness is, for example, 25 nanometers. The p-typegallium nitride semiconductor layer 35 can be a Mg-doped p⁺-type GaNcontact layer, and its thickness is, for example, 25 nanometers.

An undoped GaN layer 37 having a thickness of, for example, 15nanometers is grown on the active layer 19.

An electrode is formed above the semiconductor layers (13, 15, and 17).A first electrode (for example, anode) 41 a is formed on the contactlayer 35, and a second electrode (for example, cathode) 41 b is formedon the back side 11 b of the substrate. Light L is generated in responseto the injection of carriers into active layer 19 via these electrodes.Since the piezoelectric field in the active layer 19 is small, lightfrom the active layer 19 a can exhibit small blue shift. In addition,since the conduction band does not have a dip at the interface betweenthe light-emitting layer 19 and the gallium nitride based semiconductor31, the light-emitting device LE1 can exhibit enhanced the confinementof electron.

The nitride based semiconductor optical device LE1 may be furtherprovided with a substrate 11. The substrate 11 is made of a hexagonalsemiconductor In_(S)Al_(T)Ga_(1-S-T)N (0≦S≦1, 0≦T≦1, 0≦S+T≦1), such asGaN, InGaN, or AlGaN. The primary surface 11 a of the substrate 11extends along a plane tilting at a tilt angle β in the range of 59degrees to less than 80 degrees or greater than 150 degrees to less than180 degrees with reference to the plane that is orthogonal to the c-axis(for example, shown by vector VC2) of the hexagonal semiconductor. Thetilt angle β is substantially equal to the tilt angle α, as long as aslight tilt of the crystal axis due to strain in the light-emittinglayer 15 is neglected. Furthermore, the direction of the vector VC2 issubstantially equal to that of the vector VC1, as long as a slight tiltof the crystal axis due to strain in the light-emitting layer 15 isneglected.

The first gallium nitride based semiconductor region 13, thelight-emitting layer 15, and the second gallium nitride basedsemiconductor region 17 are arranged in the direction of a predeterminedaxis Ax (for example, the direction of the Z-axis) on the primarysurface 11 a of the substrate 11. The direction of the predeterminedaxis Ax is different from the direction of the c-axis of the substrate11.

The use of this substrate 11 facilitates to provide the light-emittinglayer 15 with plane orientation for the well layer such that thepiezoelectric field in the well layer 21 includes a component of adirection that is opposite to the direction from the second galliumnitride based semiconductor region 17 toward the first gallium nitridebased semiconductor region 13.

FIG. 4 is a diagram schematically showing the structure of a nitridebased semiconductor optical device according to an embodiment. Thenitride based semiconductor optical device LD1 is, for example, asemiconductor laser. FIG. 4 shows a coordinate system S. The primarysurface 13 a of a substrate 13 is directed to the Z-axis, and extends inthe X and Y directions. The Y-axis is orientated in the direction of them-axis.

The nitride based semiconductor optical device LD1 has a structuresuitable for a semiconductor laser, and includes a first gallium nitridebased semiconductor region 13, a light-emitting region 15, and a secondgallium nitride based semiconductor region 17. The light-emitting layer15 includes an active layer 19, and the active layer 19 has aquantum-well structure that includes a well layer 21 and a barrier layer23, which are alternately arranged. The light-emitting layer 15 isprovided between the first gallium nitride based semiconductor region 13and the second gallium nitride based semiconductor region 17. The firstgallium nitride based semiconductor region 13 may include one or moren-type gallium nitride based semiconductor layers (in this embodiment,gallium nitride based semiconductor layers 55 and 57). The secondgallium nitride based semiconductor region 17 includes a gallium nitridebased semiconductor layer 31 of a band gap larger than that of thebarrier layers, and one or more p-type gallium nitride basedsemiconductor layers (in this embodiment, gallium nitride basedsemiconductor layers 51 and 53).

In the nitride based semiconductor optical device LD1, each well layer21 extends along a reference plane SR1 tilting at a tilt angle α withrespect to the plane that is orthogonal to a reference axis (shown byvector VC1) extending in the direction of the c-axis. The tilt angle αmay be in the range of 59 degrees to less than 80 degrees.Alternatively, the tilt angle α may be in the range of greater than 150degrees to less than 180 degrees. The well layers 21 have internalstrain, and the piezoelectric fields in the strained well layers 21include components in the direction that is opposite to the directionfrom the second gallium nitride based semiconductor region 17 toward thefirst gallium nitride based semiconductor region 13. The gallium nitridebased semiconductor layer 31 of the second gallium nitride basedsemiconductor region 17 is adjacent to the light-emitting layer 15.

In this nitride based semiconductor optical device LD1, since each ofthe well layers 21 and the barrier layers 23 extends along the referenceplane SR1 tilting at a tilt angle α within the above-mentioned anglerange, the piezoelectric field in the well layers 21 includes acomponent in the direction (positive direction of the Z-axis) that isopposite to the direction from the second gallium nitride basedsemiconductor region 17 toward the first gallium nitride basedsemiconductor region 13. On the other hand, the piezoelectric field inthis gallium nitride based semiconductor layer 31 includes a componentin the direction (negative direction of the Z-axis) that is the same asthe direction from the second gallium nitride based semiconductor region17 toward the first gallium nitride based semiconductor region 13. Thegallium nitride based semiconductor layer 31 of the second galliumnitride based semiconductor region 17 is adjacent to the light-emittinglayer 15, and thus, the valence band, not the conduction band, has a dipat the interface J2 between the gallium nitride based semiconductorlayer 31 and the light-emitting layer 15. The dip thus generated in thevalence band, not in the conduction band, leads to a reduction inoverflow of electrons.

In the semiconductor light-emitting device LD1, the n-type galliumnitride based semiconductor layer 55 in the first gallium nitride basedsemiconductor region 13 can be, for example, a Si-doped n-type AlGaNcladding layer, its thickness can be, for example, 2300 nanometers andits fraction of aluminum can be, for example, 0.04. The n-type galliumnitride based semiconductor layer 55 can be, for example, a Si-dopedn-type GaN layer, and its thickness can be, for example, 50 nanometers.The light-emitting layer 15 may include first and second optical guidelayers 59 a and 59 b. The active layer 19 is provided between theoptical guide layers 59 a and 59 b. The optical guide layers 59 a and 59b may be composed of, for example, undoped InGaN, and its fraction ofindium can be, for example, 0.06 and its thickness can be, for example,100 nanometers.

The p-type gallium nitride based semiconductor layer 31 of the secondgallium nitride based semiconductor region 17 can be, for example, aMg-doped p-type AlGaN layer, its fraction of aluminum can be, forexample, 0.18 and its thickness can be, for example, 20 nanometers. Thep-type gallium nitride based semiconductor layer 51 is a Mg-doped p-typeAlGaN cladding layer, and its fraction of aluminum can be, for example,0.06. The thickness of p-type gallium nitride based semiconductor layer51 can be, for example, 400 nanometers. The p-type gallium nitride basedsemiconductor layer 53 can be a Mg-doped p⁺-type GaN contact layer, andits thickness can be, for example, 50 nanometers.

An undoped GaN layer 61 having a thickness of, for example, 50nanometers is grown on the active layer 19. An insulating film 63 havinga stripe window is formed above the semiconductor layers (13, 15, 17),and an electrode is formed on the insulating film 63 and thesemiconductor layers (13, 15, 17). A first electrode (for example,anode) 65 is formed on the contact layer 53, and a second electrode (forexample, cathode) 67 is formed on the back side 13 b of the substrate.The active layer 19 generates laser beam in response to the injection ofcarriers via these electrodes. Since the piezoelectric field in theactive layer 19 is small, the blue shift can be also small. In addition,since the conduction band does not have any dip at the interface betweenthe light-emitting layer 19 and the gallium nitride semiconductor 31,the light-emitting device LD1 can exhibit enhanced confinement ofelectron.

In the nitride based semiconductor optical devices LE1 and LD1, thereference plane SR1 may tilt toward the direction of the a-axis. Thetilt toward the direction of the a-axis enables the cleavage of them-plane. In addition, the reference plane SR1 may tilt toward thedirection of the m-axis. The tilt toward the direction of the m-axisenables the cleavage of the a-plane.

FIGS. 5 to 7 include diagrams showing primary steps in a method offabricating a nitride based semiconductor optical device according to anembodiment, and in the method of fabricating an epitaxial wafer for thisoptical device. As shown in part (a) of FIG. 5, in step S101, asubstrate 71 for forming a nitride based semiconductor optical deviceand an epitaxial wafer is prepared. The substrate 71 can be made of, forexample, a hexagonal semiconductor In_(S)Al_(T)Ga_(1-S-T)N (0≦S≦1,0≦T≦1, 0≦S+T≦1) and includes a primary surface 71 a and a back surface71 b. With reference to part (a) of FIG. 5, the vector VC indicates thedirection of the c-axis of the hexagonal semiconductor substrate 71, andthe vector VN indicating a line normal to the primary surface 71 a. Thevector VC2 indicates the direction of a {0001} plane. This substrate 71can provide the primary surface with semipolar of a tilt angle (offangle) β. The tilt angle of the primary surface 71 a of the substrate 71is in the range of greater than 59 degrees to less than 80 degrees orgreater than 150 degrees to less than 180 degrees with reference to the{0001} plane of the hexagonal semiconductor. At a tilt angle of theprimary surface 71 a of 59 degrees to less than 80 degrees or of greaterthan 150 degrees to less than 180 degrees, the piezoelectric field inthe well layer in the nitride based semiconductor optical device formedon the primary surface of the substrate 71 includes a component in adirection that is opposite to the direction from the second galliumnitride based semiconductor region toward the first gallium nitridebased semiconductor region, and the piezoelectric field in the galliumnitride semiconductor layer of the second gallium nitride basedsemiconductor region includes a component in a direction that is thesame as the direction from the second gallium nitride basedsemiconductor region toward the first gallium nitride basedsemiconductor region. Accordingly, a nitride based semiconductor opticaldevice with enhanced electron confinement can be fabricated.

The maximum distance Dia between two points on the edge of the substrate71 may be 45 millimeters or more. Such a substrate is called, forexample, a wafer. The back side 11 b of the substrate 71 may besubstantially parallel to the front side of the substrate 71. Inaddition, when the substrate 71 is made of GaN, this GaN substrateenables epitaxial growth with high crystal quality.

In the subsequent step, semiconductor crystals is epitaxially grown onthe primary surface 71 a of the substrate 71 with an off angle selectedsuch that a negative piezoelectric field is generated in the well layer.The substrate 71 having a primary surface 71 a with the above-mentionedtilt angle can form an epitaxial semiconductor region such that the welllayer in the active layer tilts from the c-plane within theabove-mentioned angle range.

Regarding the tilt direction of the primary surface 71 a of thesubstrate 71, when the primary surface 71 a tilts toward the directionof the a-axis of the hexagonal semiconductor substrate 71, the epitaxialsubstrate formed on this substrate 71 can be cleaved along the m-plane.When the primary surface 71 a of the substrate 71 tilts toward thedirection of the m-axis of the hexagonal semiconductor substrate 71, theepitaxial substrate formed on the substrate 71 can be cleaved along thea-plane. Furthermore, when the primary surface 71 a tilts toward thedirection of the a-axis of the hexagonal semiconductor substrate 71, theoff angle in the direction of the m-axis is preferably in the range of−3 degrees to +3 degrees. In addition, when the primary surface 71 atilts toward the direction of the m-axis of the hexagonal semiconductorsubstrate 71, the off angle in the direction of the a-axis is preferablyin the range of −3 degrees to +3 degrees. When the laser cavity of thenitride based semiconductor optical device LD1 has ends that tilt withinthe above ranges, a decrease in reflectivity of the ends is made small,leading to a low oscillation threshold.

The substrate 71 is loaded into a reactor 10. As shown in part (b) ofFIG. 5, in step S102, the substrate 71 is heat-treated while a gas G0 issupplied to the reactor 10, thereby forming a modified primary surface71 c prior to the growth. This heat treatment can be performed in a gasatmosphere containing ammonia and hydrogen. The heat treatmenttemperature T₀ may be in the range of, for example, from 800° C. to1200° C. The heat treatment is carried out for, for example, about 10minutes. After this step, a semipolar primary surface has a surfacestructure, which is different from that of the c-plane, formed becauseof the tilt of the primary surface 71 a. The heat treatment of theprimary surface 71 a of the substrate 71 performed in advance of thegrowth results in modification of the semiconductor primary surface,which cannot be achieved by any c-plane primary surface. The epitaxiallygrown films of gallium nitride based semiconductor will be grown on themodified primary surface 71 c of the substrate 71.

As shown in part (c) of FIG. 5, in step S103, after the heat treatment,a first conductivity-type gallium nitride based semiconductor region 73is epitaxially grown on the surface 71 c of the substrate 71. Thecrystal growth is performed by metal-organic chemical vapor depositionusing a gallium source, an indium source, an aluminum source, and anitrogen source, as raw material for semiconductor growth. The galliumsource, the indium source, and the nitrogen source are, for example,TMG, TMI, TMA, and NH₃, respectively. For the above growth, a rawmaterial gas G1 is supplied to the reactor 10. The primary surface 73 aof the gallium nitride based semiconductor region 73 tilts from thec-plane of the gallium nitride based semiconductor in an angle of therange of 59 degrees to less than 80 degrees or the range of greater than150 degrees to less than 180 degrees. The first conductivity-typegallium nitride based semiconductor region 73 a may contain one or moregallium nitride based semiconductor layers (for example, gallium nitridebased semiconductor layers 25, 27, and 29). For example, these galliumnitride based semiconductor layers 25, 27, and 29 may be an n-type AlGaNlayer, an n-type GaN layer, and an n-type InGaN layer, respectively. Thegallium nitride based semiconductor layers 25, 27, and 29 areepitaxially grown in sequence on the primary surface 71 c of thesubstrate 71. The n-type AlGaN layer 25 is formed as, for example, aninterlayer, which covers the entire surface of the substrate 71, and isgrown, for example, at 1100° C. The thickness of the n-type AlGaN layer25 is, for example, 50 nanometers. The n-type GaN layer 27 is grown onthe n-type AlGaN layer 25 at 950° C. The n-type GaN layer 27 is providedfor supplying n-type carriers, for example, and has a thickness of 2000nanometers. The n-type InGaN layer 29 is grown on the n-type GaN layer27 at 840° C. The n-type InGaN layer 29 provided for a buffer layer forthe active layer, for example, and has a thickness of 100 nanometers.

In the next step, as shown in FIGS. 6 and 7, an active layer 75 of thenitride semiconductor light-emitting device is fabricated. The activelayer 75 is provided so as to have a light-emitting spectrum having apeak wavelength in the wavelength region of 370 nanometers to 650nanometers.

As shown in part (a) of FIG. 6, in step S104, a barrier layer 77composed of a gallium nitride based semiconductor is formed for aquantum-well structure of the active layer 75. The barrier layer 77 isgrown on the buffer layer at a growth temperature T_(B) by supplying araw material gas G2 into the reactor 10. The barrier layer 77 iscomposed of In_(Y)Ga_(1-Y)N (indium composition Y: 0≦Y≦0.05, where Ydenotes the strained composition). The barrier layer 77 is formed at agrowth temperature T_(B) in the range, for example, of 700° C. to 1000°C. In this embodiment, a raw material gas G2 containing a gallium sourceand a nitrogen source is supplied to the reactor 10 to grow undoped GaNat a growth temperature T_(B). The thickness of the GaN barrier layeris, for example, 15 nanometers. Since the barrier layer 77 is grown onthe primary surface 73 a, the surface of the barrier layer 77 has thesame surface structure as that of the primary surface 73 a.

After the completion of the growth of the barrier layer 77, the supplyof the gallium source is stopped to terminate the growth of the galliumnitride based semiconductor. After the growth of the barrier layer 77and before the growth of a well layer, the temperature of the reactor ischanged from the growth temperature T_(B) to a growth temperature T_(W).During this period of time for changing the temperature, a nitrogensource gas, such as ammonia, is supplied into the reactor 10.

As shown in part (b) of FIG. 6, in step S105, a well layer 79 for thequantum-well structure is grown on the barrier layer 77, while thetemperature of the reactor 10 is maintained at a well-layer growthtemperature T_(W). The well layer 79 is composed of a gallium nitridebased semiconductor containing indium, such as In_(X)Ga_(1-X)N (indiumcomposition X: 0<X<1, X denotes strained composition), and has a bandgap energy smaller than that of the barrier layer 77. The growthtemperature T_(W) for the well layer 79 is lower than the growthtemperature T_(B). In this example, a raw material gas G3 containing agallium source, an indium source, and a nitrogen source is supplied tothe reactor 10 to grow undoped InGaN. The well layer 79 may have athickness of 1 to 10 nanometers. In addition, the indium composition Xof the In_(X)Ga_(1-X)N well layer 79 may be greater than 0.05. Theindium composition of In_(X)Ga_(1-X)N in the well layer 79 may be lessthan 0.5. InGaN having an indium composition in the above range can begrown to form a light-emitting device with a emitting wavelength of 370to 650 nanometers. The well layer 79 is grown at a growth temperatureT_(W) within the range of, for example, 600° C. to 900° C. The thicknessof the InGaN well layer is, for example, 3 nanometers. Since the primarysurface of the well layer 79 is epitaxially grown on the primary surfaceof the barrier layer 77, the surface of the well layer 79 has the samesurface structure as that of the barrier layer 77, and tilts from thec-plane of the gallium nitride based semiconductor by a predeterminedangle in association with the tilt angle of the primary surface of thebarrier layer 77.

After the growth of the well layer 79 and before the growth of anotherbarrier layer, the temperature of the reactor 10 is changed from thegrowth temperature T_(W) to the growth temperature T_(B). During thisperiod of time for changing the temperature, a nitrogen source gas, suchas ammonia, is supplied into the reactor 10. After the completion of theheating of the reactor 10, in step S106, as shown in part (c) of FIG. 6,a barrier layer 81 composed of a gallium nitride based semiconductor isgrown while a raw material gas G4 is fed into the reactor 10 in whichthe growth temperature T_(B) is maintained. In this example, the barrierlayer 81 is composed of, for example, GaN, and has a thickness of, forexample, 15 nanometers. Since the primary surface of the barrier layer81 is epitaxially grown on the primary surface of the well layer 79, andaccordingly, the surface of the barrier layer 81 has the same surfacestructure as that of the well layer 79.

In step S107, similar growth is repeatedly performed to complete anactive layer 75 having a quantum-well structure, as shown in part (a) ofFIG. 7. The active layer 75 includes three well layers 79 and fourbarrier layers 77 and 81. Then, as shown in part (b) of FIG. 7, in stepS108, a light-emitting layer 83 is formed by growing a necessarysemiconductor layer(s) by supplying a raw material gas G5. The band gapof the semiconductor layer in the light-emitting layer 83, which isprovided between the active layer 75 and a second conductivity-typegallium nitride based semiconductor region 85, is smaller than that ofthe gallium nitride based semiconductor layer, which is in contact withthe light-emitting layer 83, of the second conductivity-type galliumnitride based semiconductor region 85.

As shown in part (c) of FIG. 7, in step S109, the secondconductivity-type gallium nitride based semiconductor region 85 isepitaxially grown on the light-emitting layer 83 by supplying a rawmaterial gas G6 in the reactor 10. The second conductivity-type galliumnitride based semiconductor region 85 may include, for example, anelectron blocking layer 31, a first p-type contact layer 33, and asecond p-type contact layer 35. The electron blocking layer 31 may becomposed of, for example, AlGaN, and the p-type contact layers 33 and 35may be composed of p-type GaN. The dopant concentration N₃₇ of thesecond p-type contact layer 35 is larger than the dopant concentrationN₃₅ of the first p-type contact layer 33. In this example, the growthtemperature of the electron blocking layer 31 and the p-type contactlayers 33 and 35 is, for example, 1100° C. The second conductivity-typegallium nitride based semiconductor region 31 is formed to complete anepitaxial wafer E, which is shown in part (c) of FIG. 7. If necessary, apair of optical guide layers for optical guiding in a semiconductorlaser may be grown. These optical guide layers sandwich the activelayer, and may be composed of, for example, InGaN or GaN.

In the epitaxial wafer E, the first conductivity-type gallium nitridebased semiconductor region 73, the light-emitting layer 83, and thesecond conductivity-type gallium nitride based semiconductor layer 85may be arranged in the axial direction normal to the primary surface 71a of the substrate 71. The direction of the c-axis of the hexagonalsemiconductor is different from the direction of the axis that is normalto the primary surface 71 a of the substrate 71. The direction of theepitaxial growth is the direction of the c-axis, and is different fromthe direction in which the semiconductor layers 73, 83, and 85 arelaminated.

In the next step, electrodes are formed on the epitaxial wafer E. Afirst electrode (for example, anode) is formed on the contact layer 35,and a second electrode (for example, cathode) is formed on the back side71 b of the substrate.

After the formation of the electrodes, end faces for an optical cavitymay be created by cleavage. That is, a semiconductor laser can beproduced which utilizes these end faces for an optical cavity formed bycleavage. When the primary surface 71 a of the substrate 71 tilts towardthe direction of the a-axis of the gallium nitride based semiconductor,the m-plane can be used as a cleavage plane. When the primary surface 71a of the substrate 71 tilts toward the direction of the m-axis of thegallium nitride based semiconductor, the a-plane can be used as acleavage plane.

FIG. 8 is a diagram showing the structure of a GaN substrate that can beused in the present embodiments. The substrate 11 may include aplurality of first regions and a plurality of second regions 12 b. Thefirst regions 12 a extend in the direction of the c-axis and have athreading dislocation density larger than the first threadingdislocation density, and the second regions 12 b extend in the directionof the c-axis and have a threading dislocation density smaller than thefirst threading dislocation density. The first and second regions 12 aand 12 b reach the primary surface 11 a of the substrate 11, and thewidths of the first and second regions 12 a and 12 b in the primarysurface 11 a of the substrate 11 are, for example, 500 μm and 5000 μm,respectively. The first and second regions 12 a and 12 b are alternatelyarranged in a predetermined direction in a plane where the primarysurface extends. When the substrate is composed of gallium nitride, thepredetermined direction may be the direction of the a-axis of thegallium nitride.

The first regions 12 a are made of a high-defect-density region ofsemiconductor with a high dislocation density, whereas the secondregions 12 b are made of a low-defect-density region of semiconductorwith a low dislocation density. Forming a nitride semiconductorlight-emitting device in the low dislocation density region of thesubstrate 11 can enhance its light-emitting efficiency and reliability.When the second regions 12 b have a threading dislocation density ofless than 1×10⁷ cm⁻², the semiconductor laser using the relevantsubstrate can be provided with reliability sufficient for practical use.

EXAMPLE 1

Gallium nitride semiconductor wafers with primary surfaces havingdifferent off angles were prepared, and the directions of piezoelectricfields in light-emitting layers were estimated. FIG. 9 is a flowchartshowing the method of estimating the direction and the magnitude of thepiezoelectric field of a well layer.

In the following description, wafers of GaN were used. In step S201, inorder to estimate the direction of the piezoelectric field in alight-emitting layer, a plane orientation of the light-emitting layer isselected.

In step S202, a quantum-well structure for estimating the direction ofthe piezoelectric field in the light-emitting layer was formed so as tohave a selected plane orientation, and p-type and n-type gallium nitridesemiconductors were grown to fabricate epitaxial wafers. After thegrowth, a cathode and an anode were formed on each epitaxial wafer toproduce a substrate product.

For example, light-emitting devices having a structure shown in FIG. 1were grown on the following GaN wafers: GaN wafer with a c-plane(referred to as device name: C); GaN wafers with 75-degree-off planetoward the direction of the m-axis (referred to as device names: M75_1and M75_2), and GaN wafers with 58-degree-off plane toward the directionof the a-axis (referred to as device names: A58_1, A58_2, and A58_3). Acandidate of the above 75-degree-off plane toward the direction of them-axis is (20-21) plane, and a candidate of the above 58-degree-offplane toward the direction of the a-axis is (11-22) plane. An example ofthe structure of the substrate products:

-   Wafer: n-type GaN single crystal,-   Si-doped Al_(0.12)Ga_(0.88)N: 50 nanometers,-   Si-doped GaN layer: 2000 nanometers,-   Si-doped In_(0.02)Ga_(0.98)N layer: 100 nanometers,-   Undoped In_(0.20)Ga_(0.80)N well layer: 3 nanometers,-   Undoped GaN barrier layer: 15 nanometers,-   Mg-doped Al_(0.16)Ga_(0.84)N layer: 20 nanometers,-   Mg-doped GaN layer: 25 nanometers, and-   Heavily Mg-doped GaN layer: 25 nanometers.

In step S203, a PL measurement apparatus was prepared, and by use ofthis apparatus, PL measurement of the devices formed above can beperformed while biased voltage was applied thereto. Part (a) of FIG. 10is a schematic diagram showing an example of the structure of the PLmeasurement apparatus. The PL measurement apparatus includes anexcitation light source 93 for irradiating a device DEV with excitationlight, a PL detector 95 for detecting photoluminescence from the deviceDEV, and a section 97 for applying a variable bias to the device DEV.

In step S204, the dependency of the photoluminescence on bias wasmeasured while bias was applied to the substrate product. Part (b) ofFIG. 10 reveals that the observed results of bias dependency shown as,for example, characteristic curves. The device DEV emitselectroluminescence when it is energized with a forward bias voltageexceeding a certain level of intensity. The electroluminescence is notgenerated when the forward bias is small or is reversed.

The devices formed on a semipolar plane having an off angle within acertain range and on the c-plane of a GaN wafer exhibits the generationof a positive piezoelectric field in the light-emitting layer. Thecharacteristic curve PLB(+) of part (b) of FIG. 10 shows characteristicsof these devices. In the bias lower than the EL light-emission voltage,the peak wavelength of the PL light-emission shifts toward longerwavelengths with an increase in the bias. In the voltage over the ELlight-emission voltage, the peak wavelength shifts toward shorterwavelengths with an increase in the bias.

In the device formed on a nonpolar plane of the GaN wafer, thepiezoelectric field in the light-emitting layer is zero. Thecharacteristic curve PLB(NP) of part (b) of FIG. 10 showscharacteristics of this device. Before the bias reaches the zerovoltage, the peak wavelength of the PL light-emission slightly shiftstoward shorter wavelengths with an increase in the bias, but does notsubstantially shift at a positive bias.

The device formed on a semipolar plane having a specific off angle rangeaccording to this embodiment generates a negative piezoelectric field inthe light-emitting layer. The characteristic curve PLB(−) of part (b) ofFIG. 10 shows characteristics of this device. Before the bias reachesthe EL light-emission voltage, the peak wavelength of the PLlight-emission slightly shifts toward shorter wavelengths with anincrease in the bias.

In step 205, the direction of the piezoelectric field in thelight-emitting layer is estimated from the measured bias dependency. Thedirection of the piezoelectric field in the light-emitting layer can bedetermined based on part (b) of FIG. 10.

In step 206, a wafer is prepared which has a primary surface that canfabricate a light-emitting layer with the selected plane orientation. Instep S207, a semiconductor laminate for the semiconductor light-emittingdevice is formed on the primary surface of this wafer. As shown in FIGS.1 and 4, the semiconductor laminate can include the first galliumnitride based semiconductor region 13, the light-emitting layer 15, andthe second gallium nitride based semiconductor region 17. Thelight-emitting layer 15 includes well layers and barrier layers. Each ofthe well layers and the barrier layers extends along a reference planetilting from planes orthogonal to the reference axes that extend in thedirections of the c-axis, the a-axis, and the m-axis. The light-emittinglayer 15 is provided between the first gallium nitride basedsemiconductor region 13 and the second gallium nitride basedsemiconductor region 17. The direction of the piezoelectric field isdefined with reference to the direction that is from the second galliumnitride based semiconductor region 17 toward the first gallium nitridebased semiconductor region 13. Since the dependency of PL spectrum onbias is measured while bias is applied thereto, photoluminescence can beobtained in the range of the negative application voltage and positiveapplication voltage smaller than the application voltage at which lightby electroluminescence emits. The magnitude and the direction of theinternal electrical field in the light-emitting layer can be estimatedusing the bias dependency of photoluminescence.

FIG. 11 shows the results of observed EL spectra of semiconductorlight-emitting devices formed in an example. With reference to FIG. 11,the amount of the blue shift on the GaN c-plane (referred to as devicename: C) is about 30 nanometers at the applied current of 120 mA. Theamount of the blue shift on the 75-degree-off plane toward the m-axis(referred to as device name: M75_1 and M75_2) is about 4 to 7nanometers. The amount of the blue shift on the 58-degree-off planetoward the a-axis (referred to as device name: A58_1, A58_2, and A58_3)is about 7 to 16 nanometers.

The 75-degree-off plane in the m-direction and the 58-degrees-off planein the a-direction exhibit small blue shifts, whereas the device on thec-plane exhibits a very large blue shift. In particular, the75-degree-off plane in the m-direction can reduce blue shift. This isadvantageous in that the color tone of a light-emitting diode does notvary depending on an applied current and that a laser diode provides along wavelength oscillation.

A c-plane, a 75-degree-off plane in the direction of the m-axis, and a58-degree-off plane in the direction of the a-axis were prepared, andlight-emitting diodes (LEDs) were fabricated similarly. The LEDs wereenergized to measure EL spectra at variable temperatures of the LEDs.

As shown in FIG. 12, in terms of the dependency of the integralintensity on temperature, the LED (characteristic curve: c) on thec-plane exhibits a steep decrease in the integral intensity in thetemperature range of 150 K or less, whereas the LED (characteristiccurve: m75) on the 75-degree-off plane in the direction of the m-axisand the LED (characteristic curve: a50) on the 58-degree-off plane inthe direction of the a-axis both do not exhibit any substantial decreasein integral intensity in the low temperature range. As shown in FIG. 13,the comparison of the EL spectra reveals that each of the three LEDs(refer to three characteristic curves in FIGS. 13 as m75(300), a58(300),and c(300)) shows the single peak of emission from the onlylight-emitting layer at an absolute temperature of 300 K. On the otherhand, as shown in FIG. 14, only the LED on the c-plane exhibits anadditional peak around 380 nanometers (refer to three characteristiccurves in FIGS. 14 as m75(10), a58(10), and c(10)) at an absolutetemperature of 10 K. The additional peak indicates that electronsoverflowing from the light-emitting layer recombine with holes in thep-type layer to emit the light. That is, in the LED on the c-plane,since the activation rate of the acceptor decreases at low temperature,a dip of the conduction band at the interface between the light-emittinglayer and the p-type layer becomes large, thereby causing electrons tosignificantly overflow. This phenomenon is not observed in the LEDs onthe 75-degree-off plane in the direction of the m-axis and the LED onthe 58-degree-off plane in the direction of the a-axis, resulting infewer overflows of electrons in these LEDs.

EXAMPLE 2

A semiconductor laser LD0 having a structure shown in FIG. 15 wasfabricated. A GaN wafer 90 with an off-angle of 75 degrees in thedirection of the m-axis was prepared. The GaN wafer 90 was placed in areactor, and was heat-treated in an atmosphere containing ammonia andhydrogen. The heat treatment temperature was set at a temperature of1100° C., and the heat treatment was performed for about 10 minutes.

After the heat treatment, TMG (98.7 μmol/min), TMA (8.2 μmol/min), NH₃(6 slm), and SiH₄ were supplied to the reactor to grow an n-type AlGaNlayer 91 for a cladding layer on the GaN wafer 90 at a temperature of1150° C. The thickness of the n-type AlGaN layer 91 was 2300 nanometers,the growth rate of the n-type AlGaN layer 91 was 46.0 nm/min, and the Alcomposition of the n-type AlGaN layer 91 was 0.04.

Then, TMG (98.7 μmol/min), NH₃ (5 slm), and SiH₄ were supplied to thereactor to grow an n-type GaN layer 92 on the n-type AlGaN layer 91 at atemperature of 1150° C. The thickness of the n-type GaN layer 92 was 50nanometers, and the growth rate of the n-type GaN layer 92 was 58.0nm/min.

TMG (24.4 μmol/min), TMI (4.6 μmol/min), and NH₃ (6 slm) were suppliedto the reactor to grow an undoped InGaN layer 93 a for an optical guidelayer on the n-type GaN layer 94 at a temperature of 840° C. Thethickness of the n-type InGaN layer 93 a was 65 nanometers, the growthrate of the n-type InGaN layer 93 a was 6.7 nm/min, and the indiumcomposition of the undoped InGaN layer 93 a was 0.05.

Then, an active layer 94 was grown thereon. TMG (15.6 μmol/min), TMI(29.0 μmol/min), and NH₃ (8 slm) were supplied to the reactor to grow anundoped InGaN well layer at a temperature of 745° C. The thickness ofthe InGaN layer was 3 nanometers, and the growth rate of the InGaN layerwas 3.1 nm/min.

Then, TMG (15.6 μmol/min), TMI (0.3 μmol/min), and NH₃ (8 slm) weresupplied to the reactor, while the temperature of the reactor wasmaintained at a temperature of 745° C., to grow an undoped GaN layer onthe InGaN layer. The thickness of the GaN layer was one nanometer, andthe growth rate of the GaN layer was 3.1 nm/min. After the growth of theundoped GaN layer, the temperature of the reactor was changed from 745°C. to 870° C., and TMG (24.4 μmol/min), TMI (1.6 μmol/min), and NH₃ (6slm) were supplied to the reactor to grow an undoped InGaN layer for abarrier layer on the undoped InGaN well layer at a temperature of 870°C. The thickness of the InGaN layer was 15 nanometers, and the growthrate of the InGaN layer was 6.7 nm/min. The In composition of theundoped InGaN layer was 0.02.

Then, the temperature of the reactor was changed from 870° C. to 745°C., and TMG (15.6 μmol/min), TMI (29.0 μmol/min), and NH₃ (8 slm) weresupplied to the reactor to grow an undoped InGaN well layer on the InGaNlayer at a temperature of 745° C. The thickness of the InGaN layer was 3nanometers, and the growth rate of the InGaN layer was 3.1 nm/min. TheIn composition of the undoped InGaN layer was 0.25.

The growth of the well layer, the protective layer, and the barrierlayer were repeated twice, and a well layer and a protective layer werefurther grown thereon. Then, TMG (13.0 μmol/min), TMI (4.6 μmol/min),and NH₃ (6 slm) were supplied to the reactor to grow an undoped InGaNlayer 93 b for an optical guide layer on the active layer 94 at atemperature of 840° C. The thickness of the InGaN layer 93 b was 65nanometers, and the growth rate of the InGaN layer 93 b was 6.7 nm/min.Then, TMG (98.7 μmol/min) and NH₃ (5 slm) were supplied to the reactorto grow an undoped GaN layer 96 on the InGaN layer 93 b at a temperatureof 1100° C. The thickness of the GaN layer 96 was 50 nanometers, and thegrowth rate of the GaN layer 96 was 58.0 nm/min. The In composition ofthe undoped InGaN layer 93 b was 0.05.

Then, TMG (16.6 μmol/min), TMA (2.8 μmol/min), NH₃ (6 slm), and Cp₂Mgwere supplied to the reactor to grow a p-type AlGaN layer 97 on the GaNlayer 96 at a temperature of 1100° C. The thickness of the AlGaN layer97 was 20 nanometers, and the growth rate of the AlGaN layer 97 was 4.9nm/min. The Al composition of the p-type AlGaN layer 97 was 0.15.

TMG (36.6 μmol/min), TMA (3.0 μmol/min), NH₃ (6 slm), and Cp₂Mg weresupplied to the reactor to grow a p-type AlGaN layer 98 on the p-typeAlGaN layer 97 at a temperature of 1100° C. The thickness of the AlGaNlayer 98 was 400 nanometers, the Al composition was 0.06, and the growthrate of the AlGaN layer 98 was 13.0 nm/min. Furthermore, TMG (34.1μmol/min), NH₃ (5 slm), and Cp₂Mg were supplied to the reactor to grow ap-type GaN layer 99 on the p-type AlGaN layer 98 at a temperature of1100° C. The thickness of the GaN layer 99 was 50 nanometers, and thegrowth rate of the p-type GaN layer 99 was 18.0 nm/min. Through thesesteps, an epitaxial wafer was fabricated. An anode and a cathode wereformed on this epitaxial wafer to provide a semiconductor diode as shownin the drawing. The anode was electrically connected to the p-type GaNlayer via a stripe window with a width of 10 μm which the insulatingfilm has. The anode was composed of Ni/Au, and the cathode was composedof Ti/Al/Ti/Au. By a-plane cleavage, a laser bar with a length of 600 μmwas formed. The lasing wavelength was 520 nanometers, and the thresholdcurrent was 900 mA.

Having described and illustrated the principle of the invention in apreferred embodiment thereof, it is appreciated by those having skill inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. The present invention is notlimited to the specific configurations described in the embodiments. Wetherefore claim all modifications and variations coming within thespirit and scope of the following claims.

1. A nitride based semiconductor optical device comprising: a firstgallium nitride based semiconductor region; a light-emitting layerincluding a well layer and a barrier layer, the well layer beingcomposed of a strained hexagonal gallium nitride based semiconductor,and the barrier layer being composed of gallium nitride basedsemiconductor; and a second gallium nitride based semiconductor region,the light-emitting layer being provided between the first galliumnitride based semiconductor region and the second gallium nitride basedsemiconductor region; the first gallium nitride based semiconductorregion including one or more n-type gallium nitride semiconductorlayers; the second gallium nitride based semiconductor region includinga gallium nitride based semiconductor layer and one or more p-typegallium nitride semiconductor layers, the gallium nitride basedsemiconductor layer having a band gap larger than that of the barrierlayer; each of the well layer and the barrier layer extending along areference plane, the reference plane tilting at a tilt angle in therange of 59 degrees to less than 80 degrees and greater than 150 degreesto less than 180 degrees from a plane orthogonal to a reference axis,the reference axis extending in the direction of a c-axis; apiezoelectric field in the light-emitting layer including a component ofa direction opposite to a direction from the second gallium nitridebased semiconductor region toward the first gallium nitride basedsemiconductor region; the gallium nitride based semiconductor layer ofthe second gallium nitride based semiconductor region being adjacent tothe light-emitting layer; and the gallium nitride based semiconductorlayer of the second gallium nitride based semiconductor region includingone of an electron blocking layer and a cladding layer.
 2. The nitridebased semiconductor optical device according to claim 1, wherein thewell layer comprises InGaN; and the barrier layer comprises one of GaNand InGaN.
 3. The nitride based semiconductor optical device accordingto claim 1, wherein the tilt angle is in the range of 62 degrees to lessthan 80 degrees.
 4. The nitride based semiconductor optical deviceaccording to claim 1, wherein the tilt angle is in the range of greaterthan 150 degrees to 170 degrees.
 5. The nitride based semiconductoroptical device according to claim 1, further comprising a substrate of ahexagonal semiconductor of In_(S)Al_(T)Ga_(1-S-T)N (0≦S≦1, 0≦T≦1,0≦S+T≦1), wherein a primary surface of the substrate extends along areference plane tilting from a plane orthogonal to a c-axis of thehexagonal semiconductor at a tilt angle in a range of 59 degrees to lessthan 80 degrees and greater than 150 degrees to less than 180 degrees;and the first gallium nitride based semiconductor region, thelight-emitting layer, and the second gallium nitride based semiconductorregion are arranged in a direction of a predetermined axis on theprimary surface of the substrate.
 6. The nitride based semiconductoroptical device according to claim 5, wherein the substrate includesplural first regions and plural second regions, the first regions extendin a direction of the c-axis and have a threading dislocation densitylarger than a first threading dislocation density, and the secondregions extend in the direction of the c-axis and have a threadingdislocation density smaller than the first threading dislocationdensity; the first and second regions are alternately arranged; and thefirst and second regions reach the primary surface of the substrate. 7.The nitride based semiconductor optical device according to claim 6,wherein the second regions have a threading dislocation density of lessthan 1×10⁷ cm⁻².
 8. The nitride based semiconductor optical deviceaccording to claim 5, wherein the first gallium nitride basedsemiconductor region, the light-emitting layer, and the second galliumnitride based semiconductor region constitute a semiconductor laminateon the primary surface of the substrate; the substrate has an electricalconductivity; and the hexagonal nitride based semiconductor opticaldevice further comprises a first electrode provided on the semiconductorlaminate and a second electrode provided on a back side of thesubstrate.
 9. The nitride based semiconductor optical device accordingto claim 1, wherein the light-emitting layer includes first and secondoptical guide layers and an active layer having a quantum-wellstructure; the quantum-well structure includes the well layer and thebarrier layer; and the active layer is provided between the firstoptical guide layer and the second optical guide layer.
 10. The nitridebased semiconductor optical device according to claim 1, wherein thereference plane tilts toward a direction of an a-axis.
 11. The nitridebased semiconductor optical device according to claim 1, wherein thereference plane tilts toward a direction of an m-axis.
 12. The nitridebased semiconductor optical device according to claim 1, wherein thegallium nitride based semiconductor layer of the second gallium nitridebased semiconductor region comprises p-type Al_(X)Ga_(Y)In_(1-X-Y)N(0<X≦1, 0≦Y≦1, 0<X+Y≦1) containing at least aluminum.
 13. An epitaxialwafer for a nitride based semiconductor optical device, comprising: afirst gallium nitride based semiconductor region; a light-emitting layerincluding a strained well layer and a barrier layer, the well layerbeing composed of a hexagonal gallium nitride based semiconductor, andthe barrier layer being composed of a gallium nitride basedsemiconductor; a second gallium nitride based semiconductor region; anda wafer of a hexagonal semiconductor of In_(S)Al_(T)Ga_(1-S-T)N (0≦S≦1,0≦T≦1, 0≦S+T≦1), the light-emitting layer being provided between thefirst gallium nitride based semiconductor region and the second galliumnitride based semiconductor region on the wafer; the first galliumnitride based semiconductor region including one or more n-type galliumnitride semiconductor layers; the second gallium nitride basedsemiconductor region including a gallium nitride based semiconductorlayer and one or more p-type gallium nitride based semiconductor layers,the gallium nitride based semiconductor layer having a band gap largerthan that of the barrier layer; each of the well layer and the barrierlayer extending along a reference plane, the reference plane tilting ata tilt angle in a range of one of 59 degrees to less than 80 degrees andgreater than 150 degrees to less than 180 degrees from a planeorthogonal to a reference axis, the reference axis extending in adirection of a c-axis; a piezoelectric field in the light-emitting layerincluding a component of a direction opposite to a direction from thesecond gallium nitride based semiconductor region toward the firstgallium nitride based semiconductor region; the gallium nitride basedsemiconductor layer of the second gallium nitride based semiconductorregion being adjacent to the light-emitting layer; the gallium nitridebased semiconductor layer of the second gallium nitride basedsemiconductor region including one of an electron blocking layer and acladding layer; and the barrier layer comprising one of GaN and InGaN.14. The epitaxial wafer according to claim 13, wherein the primarysurface of the wafer extends along a plane tilting from a planeorthogonal to the c-axis of the hexagonal semiconductor at a tilt anglein a range of 59 degrees to less than 80 degrees and greater than 150degrees to 170 degrees.
 15. The epitaxial wafer according to claim 13,wherein a maximum distance between two points on an edge of the wafer is45 millimeters or more.
 16. The epitaxial wafer according to claim 13,wherein the wafer is composed of electrically conductive GaN.
 17. Theepitaxial wafer according to claim 13, wherein the first gallium nitridebased semiconductor region, the light-emitting layer, and the secondgallium nitride based semiconductor region are arranged in apredetermined axial direction on the primary surface of the wafer; and adirection of the reference axis is different from a predetermined axialdirection.
 18. The epitaxial wafer according to claim 13, wherein thetilt angle is in the range of 62 degrees to less than 80 degrees. 19.The epitaxial wafer according to claim 13, wherein the tilt angle is inthe range of greater than 150 degrees to 170 degrees.
 20. (canceled)